(1) Field of the Invention
The present invention relates to an exchange system having an originating call restriction function, and more particularly to a call restriction priority control system in an exchange for restricting originating calls from accommodated subscribers and incoming calls from other exchanges.
An exchange executes a desired exchange process for originating calls from accommodated subscribers and incoming calls from other exchanges connected through transmission lines. When these originating calls or incoming calls increase to a quantity more than a previously estimated quantity, the exchange falls into an overload state so that connecting times are increased and the rate of complete connections is lowered.
To prevent such lowering of exchange services, supervision of the load on the exchange has been conducted so as to restrict originating calls from subscribers or to restrict acceptance of incoming calls from other exchanges.
Further, the present invention relates to a call restricting system for use in an abnormal congestion state in an exchange, and more particularly, relates to a call restricting system in which the number of originating calls which are subject to detection is adjusted in accordance with the level of the load.
During abnormal congestion in which loads are concentrated at the exchange, delays of dial tones, lack of receivers, etc., causes an increase of non-productive or ineffective processes so that the throughput of the exchange, i.e., the number of calls that can be processed within a predetermined period, is lowered.
When such an abnormal congestion occurs, it is required that the throughput be improved for as long as possible to improve the service.
The present invention also relates to an originating call restriction system in which the throughput of the system is improved by providing a LIFO (Last In First Out) queue common to the system for postponing a call.
In an electronic exchange system of a telephone or data exchange storage program system, the number of resources for exchange processes (call processing memory blocks, communication paths, signal receiving units, etc.) is limited. Therefore, when there is an overload, the throughput is lowered due to process delays depending on the congestion of the exchange processing resources.
It has been desired to provide an originating call restriction system which can improve the throughput for as long as possible, even during such congestion of the exchange processing resources.
(2) Description of the Related Art
As prior arts of exchange systems having a call restriction function, Japanese Unexamined Patent Publication No. 61-70844 and Japanese Unexamined Patent Publication No. 61-191153 are known.
The former reference (JPP 61-70844) discloses a call control system in an exchange in which the number of calls that can be accepted is corrected in accordance with the number of accepted calls.
The latter reference (JPP 61-191153) discloses a dynamic overload control system in an exchange, in which the number of calls that can be accepted is determined for respective kinds of calls and the respective kinds of calls are separately restricted in accordance with a comparison with the number of originating calls and the number of calls that can be accepted, respectively.
These references, however, do not distinguish originating calls from accommodated subscribers and incoming calls from other exchanges. The resources occupied by incoming calls from other exchanges, however, are much greater than those of the originating calls from the subscribers. Therefore, if the incoming calls are restricted, the resources which have been occupied up to that time become ineffective or on hold so that there was the worry that the efficiency of the communication would decrease.
Further, by the method in which a detection of a call from a subscriber is stopped when an abnormal congestion occurs as disclosed in the above references, the load on the exchange can be greatly decreased, however, since the calls from subscribers are not accepted at all for a predetermined period, services for subscribers are lowered.
Further, such a conventional system tends to effect an over restriction so that the ability of the exchange is suppressed without sufficient performance even when many communication requirements are present.
Conventionally, as a queue for postponing a call exchange resources are congested, a FIFO (First In First Out) queue is employed to postpone a call with respect to the individual resources (see, for example, 10th International Traffic Conference (ITC-10), Session 2.4, paper #5 "CUSTOMER BEHAVIOR AND UNEXPECTED DIAL TONE DELAY", and ITC-10, Session 5.2, paper #4 "PERFORMANCE ANALYSIS OF A NEW OVERLOAD STRATEGY"). Accordingly, in this case, the call which was queued first is processed first when the resource becomes free so that the postpone time for each call, i.e., the process delay time, is averaged.
In the dial tone connection process at the time of accepting an originating call, however, many subscribers tend to dial without confirming the dial tones transmitted from the exchange. Due to such behavior of the subscribers, the longer the delay of the dial tone transmission, the greater the probability the subscribers will dial before the dial tone is transmitted. Accordingly, the probability of partial dialing of a call in which only a part of the dial number can be received is increased so that incomplete calls are increased.
Such phenomena are conspicuous at a time of overload in which there are more originating calls than a calculated standard of installations. When the delay time of the dial tone becomes long, partial dialing is increased. When the processes cannot be effected because of the partial dialings, recalls are increased, and further a vicious cycle is caused by the delays of the dial tones and the increase of the partial dialings, resulting in an extremely bad system throughput (the ratio of the total calls to the completed calls).